The recovery of oil and gas from underground geological formations is of great importance in modern society which uses vast amounts of fossil fuels for its essential energy. The individual well productivity declines over a period of time because of a number of factors including changes in reservoir fluid characteristics, depletion of reservoir energy, decreasing permeability of the formation to the oil, the gradual dissipation of the expanding pressure transient, contamination of the well bore, reduced permeability of the oil through the region immediately surrounding the well bore and reduction of the internal diameter of the well pipe.
The response to the declining productivity was the development of numerous techniques which has become collectively known as well workover and stimulation. The concept of fracturing or formation breakdown has been recognized to play a very important role in the application of these oil production enhancement techniques including stimulation, acidizing, water injection and cementing of the formation.
Hydraulic fracturing has found wide usage as a well stimulation procedure for creating deep-penetrating fractures (both horizontal and vertical) that provide high capacity channels for flow from deep within the producing formation to the well as well as for overcoming damaged matrix permeability surrounding a wellbore. In order to produce gas or liquids from a well at a higher rate following a hydraulic fracturing treatment, the reservoir must contain enough fluids in place and the formation must not have regions of severe permeability reduction particularly in regions adjacent to the well. Early experimental work in shallow wells demonstrated that a hydraulically formed fracture tends to heal--that is, to lose its fluid carrying capacity after the parting pressure is released--unless the fracture is propped. Typical propping agents for retaining the integrity of the fractures are nutshells, plastic beads, aluminum spacers, glass beads, sand and urea prills.
Proppants thus provide a means for meeting the objective of the fracturing which is to increase the well production by preventing collapse of the formation and resultant decrease in fluid permeability.
Another cause of declining well production is caused by paraffin deposition from the crude oil onto the inner walls of the production tubing and equipment.
Paraffin is a reservoir produced group of straight-chain alkanes that contain more than 15 to more than 80 carbon atoms. The melting point of the paraffin increases as the size of the molecule increases. Paraffin is deposited in the form of crystalline solids which may collect on the interior of the tubing and flowlines, slowly choking off production. Paraffin deposits have also caused the breaking of pump rods. In some cases, paraffin deposits have caused plugging of formations during stimulation treatments. Paraffin has also been blamed for the difficulty in pumping crude oil at cool temperatures.
One method of handling paraffin deposition is to mechanically remove the paraffin. There are several mechanical methods for removing deposited paraffin from tubing, flowlines and pipelines which include rod scrapers, free-floating pistons, etc. The major advantage of mechanically removing paraffin is that positive cleaning is assured, however, it is limited due to time and equipment involved, costly and has the danger and difficulty inherent in retrieving tools lost in the hole during the cleaning operation.
Other methods of cleaning include:
(1) thermal methods, using bottomhole heaters, circulation of hot oil, water or steam, and heat-liberating chemicals; and, chemical including the use of paraffin solvents, dispersants and detergents and crystal modifiers whereby the latter prevents paraffin deposition by disrupting the nucleation, agglomeration and/or deposition of the paraffin crystals environment. At present the chemical reagent is injected into the desired location but it can be difficult to supply it uniformly to the optimum location, for instance in the permeable, oil bearing, mineral formation. Also it is necessary to inject the reagent continuously or repeatedly since it is soluble in the produced fluid and so is rapidly removed from the point of injection.
As indicated it is known to introduce reagents downhole during fracturing and other well stimulation processes. Traditionally this is done by forcing a solution of the reagent down the hole and into the formation, whereupon it becomes absorbed onto the formation and is released slowly from it. Unfortunately the rate of release is variable and generally is quite fast.
It is known to force beads of ethylene-vinyl acetate copolymers into the formation, but they are generally too large to get into the fractures formed in the formation and smaller beads would dissolve too rapidly. Also the beads are rather soft.
Another approach to overcoming the paraffin deposition in the recovery of crude oil is by adding to the oil a polymer having pendant polar and non-polar moieties, such as a partially hydrolyzed ethylene-vinyl acetate copolymer, whereby the deposition of wax from the oil is inhibited (see U.S. Pat. No. 3,693,720 wherein the inhibitor is added to the crude petroleum oil before the temperature of said oil decreases to a wax-deposition temperature).
It is known from British patent specification No. 1,290,554 to inhibit scale formation downhole by supplying downhole a solid linear carboxylic polymer having low molecular weight and in which the carboxylic groups are neutralized by an alkaline earth or other insolubilizing cation to an extent such that the polymer has a controlled low solubility in water. It is stated in that specification that water soluble scale inhibitors may also be supplied downhole with the substantially insoluble polymer.
It is an object of this invention to provide a method for using an article to enhance the production of hydrocarbons from geological reservoirs, more particularly from fractured formations.
It has been an additional object to devise a method for providing controlled release of a reagent downhole, in a pipeline, in other oil-containing environments or fluids.